Many aircraft are designed to fly at high altitudes, e.g., from 10,000 feet (about 3,000 m) to upwards of 41,000 feet (about 12,500 m), while providing a safe, comfortable cabin environment. To maintain this cabin environment, a typical aircraft includes an air conditioning system, with appropriate pressure, temperature, and moisture regulation, to circulate fresh air within the passenger cabin. An aircraft air conditioning system is sometimes referred to as an air conditioning pack, an air handling system, and/or an air circulation system. An air conditioning system may circulate outside air mixed with an approximately equal amount of highly filtered air from the passenger cabin. The combined outside and filtered air is ducted to the cabin and distributed to cabin outlets throughout the cabin, typically louvers, air distribution rails, vents, and personal air outlets (e.g., eyeball gaspers above passenger seating). Air diffusers direct the ducted air into the cabin outlets.
Inside the cabin, air diffusers and outlets are generally arranged along the side walls of the cabin and sometimes along the overhead. The air flows in generally circular patterns and exits through grilles, often on either side of the cabin floor, and, on airplanes with overhead recirculation, the air may exit through overhead grilles. For commercial aircraft, the FAA requires a minimum air flow and cabin pressure. For new aircraft, the minimum air flow is 0.55 lbs/min per occupant (about 250 g/min (grams per minute) per occupant) and the minimum cabin pressure is 0.75 bar (75 kPa).
The cabin air flow is continuous and is used for maintaining a comfortable cabin temperature, pressurization, and/or overall air quality. About half of the air exiting the cabin is exhausted from the airplane through one or more outflow valves in the fuselage, which also controls the cabin pressure. The other half is drawn through high efficiency filters, and then is recirculated with fresh outside air.